Le chÂtelier’s principle

Le chÂtelier’s principle “when a system at equilibrium is disturbed by a change in temp., pressure, or conc., the system shifts its equilibrium to counteract this change.” new equilibrium C  D  A  old equilibrium B added to system

Stresses on Equilibrium **Catalyst has no effect on equilibrium** Concentration Adding or removing a reactant or product Temperature Heating or cooling a reaction Dependent on if reaction is endo or exo Pressure Depends ONLY on gases in a reaction A shift is when a stress is applied to a reaction and the system corrects the change to return to equilibrium

Change in concentration If concentration (or amount) of a substance increases, then the reaction shifts away from the increase If concentration (or amount) of a substance decreases, then the reaction shifts towards the decrease

Concentration Change example N2(g) + 3H2(g) ↔ 2NH3(g) + 92kJ Change: H2 is added to the system What must be done to both sides of the reaction to go back to equilibrium? Shifts to side below equilibrium line: Shifts RIGHT towards products Side with H2 lifts up Equilibrium Shifts to side of rxn that must increase Shifts away from side of rxn that must decrease

Try this: N2(g) + 3H2(g) ↔ 2NH3(g) + 92kJ Change: NH3 is added to the system What must be done to both sides of the reaction to go back to equilibrium? Shifts to side below equilibrium line: Shifts LEFT towards reactants Side with NH3 lifts up Equilibrium Shifts to side of rxn that must increase Shifts away from side of rxn that must decrease

Change in TEMPERATURE Endothermic: A + B + energy  C + D If temp increases, then the reaction shifts away from the increase, towards products If temp decreases, then the reaction shifts towards the decrease, towards reactants Exothermic: A + B  C + D + energy If temp increases, towards reactants If temp decreases, towards products

Temperature Change example N2(g) + 3H2(g) ↔ 2NH3(g) + 92kJ Change: system is cooled What must be done to both sides of the reaction to go back to equilibrium? Shifts to side below equilibrium line: Shifts RIGHT towards products Equilibrium Side with heat drops Shifts to side of rxn that must increase Shifts away from side of rxn that must decrease

Try this: N2(g) + 3H2(g) ↔ 2NH3(g) + 92kJ Change: system is heated What must be done to both sides of the reaction to go back to equilibrium? Shifts to side below equilibrium line: Shifts LEFT towards reactants Side with heat lifts up Equilibrium Shifts to side of rxn that must increase Shifts away from side of rxn that must decrease

Change in Pressure Only affects gases because affects volume of system Need to count # of moles of gases (coefficients) Increase in pressure means decrease in volume, so the reaction will shift to the side with fewer moles of gas Decrease in pressure means an increase in volume, so the reaction will shift to the side with more moles of gas

Pressure change example 1 + 3 = 4 moles 2 moles N2(g) + 3H2(g) ↔ 2NH3(g) + 92kJ Change: system pressure is decreased What must be done to both sides of the reaction to go back to equilibrium? Shifts to side with more moles of gases Shifts LEFT towards reactants Reactants must increase & products must decrease to return to equilibrium V increases Shifts to side of rxn that must increase Shifts away from side of rxn that must decrease

Pressure change example 1 + 3 = 4 moles 2 moles N2(g) + 3H2(g) ↔ 2NH3(g) + 92kJ Change: system volume is decreased What must be done to both sides of the reaction to go back to equilibrium? Shifts to side with less moles of gases Shifts RIGHT towards products Reactants must decrease & products must increase to return to equilibrium P increases Shifts to side of rxn that must increase Shifts away from side of rxn that must decrease

H2 (g) + CO2 (g) + energy ↔ H2O (g) + CO (g) Try this one: Predict how each of the following will affect the equilibrium: H2 (g) + CO2 (g) + energy ↔ H2O (g) + CO (g) Adding H2O to the system Removing CO from the system Adding H2 to the system Adding a catalyst Removing energy Increase the volume of the system Nothing b/c only affects speed not reaction Nothing b/c equal # of moles

Same one but in Table form: Predict how each of the following will affect the equilibrium: H2 (g) + CO2 (g) + energy ↔ H2O (g) + CO (g) Stress Direction of Shift Effect on [H2] Effect on [CO2] Effect on [H2O] Effect on [CO] Adding H2O ------ Removing CO Adding H2 Add catalyst Removing energy Increase volume LEFT RIGHT RIGHT No Change LEFT No Change